CN114764151A - Magnetotelluric frequency division tomography inversion method - Google Patents
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Abstract
The invention relates to the technical field of geophysical inversion, in particular to a magnetotelluric frequency division tomography inversion method. The method comprises the following steps: estimating effective exploration depth according to the observation point frequency point data; grouping the frequency point data; establishing a gridding model; constructing an initial resistivity model; frequency division chromatography inversion; and (4) evaluating inversion results. According to the method, the magnetotelluric data are grouped according to frequency and inverted layer by layer from a shallow layer to a deep layer, so that the inversion unsuitability is improved, the magnetotelluric data, particularly effective information of low-frequency data, is fully mined, the deep layer resolution is improved, and the problem of low deep layer resolution of the conventional magnetotelluric inversion is solved.
Description
Technical Field
The invention relates to the technical field of geophysical inversion, in particular to a magnetotelluric frequency division tomography inversion method.
Background
The magnetotelluric exploration technology has large detection depth, is not shielded by high-resistance and high-speed strata, is quick and cheap in data acquisition and processing, and plays an important role in the fields of oil-gas exploration, regional geological survey and the like for a long time. Magnetotelluric inversion is a process of solving a resistivity model of an underground space under the constraint of empirical information and geological rules by utilizing earth surface observation data, and then interpreting an underground structure by utilizing the resistivity model. Magnetotelluric inversion is usually performed in the frequency domain with observed data in the frequency range of about 10-4-104Hz, oil and gas exploration is generally 10-3-103And (3) taking 30 to 80 frequency points between Hz, wherein the frequency point data from high to low contains information from shallow to deep underground, establishing a nonlinear equation set by using the frequency data, and inverting to obtain the process of solving the equation set.
However, the equation set is a sick equation set, and firstly, the equation set is an underdetermined equation set because the frequency point data quantity is far smaller and the inversion grid quantity is large; the second is because the sensitivity of the equation to different frequency data is different. The first reason causes the inversion to have multi-solution, and the second reason causes the information contained in the data with low sensitivity to be lost when different frequency data participate in the inversion simultaneously. The medium-high frequency data is sensitive to the medium-shallow layer structure and does not contain deep layer structure information; the low-frequency data has large detection depth and contains all construction information from shallow to deep, so that the partial undercharacterization of the corresponding middle-deep layer in an inversion equation set is strong, the partial undercharacterization of the corresponding middle-shallow layer construction is weak, if all frequency inversion is used at the same time, the inversion effect of a shallow layer model is good generally, the sensitivity of a deep layer model to data is low, the ratio of the low-frequency data in a target function is lowered by high-frequency data, the effective information is difficult to recover, and the resolution and the reliability of the deep layer model are low.
The Chinese invention patent CN104375195B discloses a time-frequency electromagnetic multi-source multi-component three-dimensional joint inversion method, which determines a time-frequency electromagnetic three-dimensional inversion initial resistivity model according to known resistivity logging data and seismic exploration data, calculates the maximum and minimum coordinates in the horizontal direction x and y, determines the inversion range of the three-dimensional inversion in the horizontal direction, selects the vertical direction grid size of the time-frequency electromagnetic multi-source multi-component three-dimensional inversion, calculates the primary field of a time-frequency electromagnetic emission source in the initial model, and calculates the Green tensor between each hexahedron underground; and calculating the derivative of each time-frequency electromagnetic field source, minimizing the target function by adopting a conjugate gradient iterative algorithm, and finishing multi-field source multi-component time-frequency electromagnetic three-dimensional inversion after iteration. The resistivity three-dimensional distribution of the underground medium of the work area is obtained after the actual measurement data is processed, and the explanation requirements of structure, fault and trap are met. But still does not solve the problem of low resolution and reliability of the deep model.
The Chinese patent application CN111983689A discloses a near-source electromagnetic seismoelectric joint GR inversion method, which comprises the following steps: acquiring electrical data of a target area by using a near-source transient electromagnetic method; acquiring drilling and logging data and seismic data of a target area; the drilling and logging data comprise logging resistivity L and a natural gamma logging curve GR, and the seismic data comprise seismic wave impedance; processing the electrical data, the seismic data and the drilling and logging data to obtain seismoelectric combined resistivity; fitting the logging resistivity L and the natural gamma logging curve GR nonlinearly by using a neural network algorithm to obtain a first relation; and performing big data joint GR inversion by using the seismoelectric joint resistivity and the seismic wave impedance according to the first relation to obtain a high-resolution GR profile of the target area. Although the method obtains the GR profile with high resolution of the target area, the steps of the method are complex.
Based on the above analysis, there is still a need for a magnetotelluric inversion method that can improve the resolution of deep layers.
Disclosure of Invention
The magnetotelluric data are grouped according to frequency and inverted layer by layer from a shallow layer to a deep layer, so that inversion unsuitability is improved, effective information of the magnetotelluric data, particularly low-frequency data, is fully mined, the resolution of the deep layer is improved, and the problem of low resolution of the deep layer of the conventional magnetotelluric inversion is solved.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides a magnetotelluric frequency division tomography inversion method, which comprises the following steps: estimating effective exploration depth according to the observation point frequency point data; grouping the frequency point data; establishing a gridding model; constructing an initial resistivity model; frequency division chromatography inversion; and (4) evaluating an inversion result.
Further, the effective exploration depth is estimated for all frequency point data of all observation points according to the following formula:
where ρ is the apparent resistivity and f is the frequency corresponding to the apparent resistivity.
Further, grouping the frequency point data: firstly, according to the effective exploration depth and the determined inversion depth, frequency point data are divided into two groups of data in an inversion region and data outside the inversion region; and secondly, grouping the data in the inversion region according to the effective exploration depth.
Furthermore, in the process of establishing the gridding model, the grid of the inversion target area is divided, the two-dimensional inversion division grid is rectangular, the three-dimensional inversion division grid is an upright hexahedron, the grid can be uniform or non-uniform, and the relief topography can be encrypted and divided at the earth surface.
Further, the number of a certain group of data is set to be m, the number of meshes split from the effective exploration depth corresponding to the previous group of data with a higher frequency than the group of data to the effective exploration depth corresponding to the group of data is set to be n, and m and n should satisfy that m/n is greater than 0.2.
Further, the method for constructing the initial resistivity model comprises the following steps: filling the same constant resistivity for each grid, and directly taking the uniform resistivity model as an initial resistivity model; alternatively, the first and second electrodes may be,
and (3) carrying out inversion by using data outside the inversion region, taking the inversion result as an initial resistivity model of the next step, and adding prior information to carry out constrained inversion in the step.
Further, the frequency division tomography inversion comprises the following specific steps:
1) first round of tomographic inversion: inputting a highest frequency data set and an initial resistivity model, and performing inversion to obtain a first round of chromatographic inversion model;
2) second round of tomographic inversion: inputting an adjacent lower frequency data group and a previous chromatographic inversion model, and inverting to obtain a new chromatographic inversion model by taking the grid resistivity in the effective exploration depth of the previous group of data as constraint;
3) and (3) repeating the step 2) until all the grouped data are inverted, and obtaining a final inversion model, namely a resistivity model.
Further, evaluating an inversion result according to the previous verification data, and if the inversion result meets the previous verification data and a satisfactory resolution is obtained, ending the inversion; otherwise, the frequency data packet is readjusted to continue inversion.
Compared with the prior art, the invention has the following beneficial effects:
according to the method, the magnetotelluric data are grouped according to frequency and inverted layer by layer from a shallow layer to a deep layer, so that the inversion unsuitability is improved, the magnetotelluric data, particularly effective information of low-frequency data, is fully mined, the deep layer resolution is improved, and the problem of low deep layer resolution of the conventional magnetotelluric inversion is solved.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
FIG. 1 is a graph of results obtained using a conventional inversion method;
fig. 2 is a diagram of an inversion result of the magnetotelluric frequency division tomography inversion method according to an embodiment of the present invention.
Detailed Description
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of the stated features, steps, operations, and/or combinations thereof, unless the context clearly indicates otherwise.
In order to make the technical solutions of the present invention more clearly understood by those skilled in the art, the technical solutions of the present invention will be described in detail below with reference to specific embodiments.
Example 1
A magnetotelluric frequency division tomography inversion method comprises the following steps:
step 1, estimating effective exploration depth according to observation point frequency point data
The depth reached when most of the energy (about 63%) of the electromagnetic wave is absorbed becomes the skin depth, which is estimated by the formula:
where ρ is the apparent resistivity and f is the frequency corresponding to the apparent resistivity. The depth at which the electromagnetic wave energy is attenuated to 50% becomes the effective exploration depth, and the estimation formula is as follows:
and estimating the effective exploration depth of all frequency point data of all observation points according to the formula.
Step 2, grouping the frequency point data:
firstly, according to the effective exploration depth and the determined inversion depth, frequency point data are divided into two groups of data in an inversion region and data outside the inversion region; secondly, the data in the inversion region are grouped according to the effective exploration depth, the basic idea of grouping is to divide the data into a shallow group, a medium group and a deep group, different grouping frequency points can have overlapping parts, and the grouping depth can determine the optimal scheme through multiple experiments. Each packet data can be further subdivided according to this concept.
Step 3, establishing a gridding model
And (3) mesh generation of the inversion target area, wherein the two-dimensional inversion generation mesh is rectangular, the three-dimensional inversion generation mesh is an upright hexahedron, the mesh can be uniform or non-uniform, and undulating terrain can be depicted in an encrypted generation manner at the earth surface. And setting the number of certain group of data as m, and setting the number of meshes divided from the effective exploration depth corresponding to the previous group of data with higher frequency than the group of data to the effective exploration depth corresponding to the group of data as n, wherein a satisfactory inversion effect can be obtained when m/n is greater than 0.2.
Step 4, constructing an initial resistivity model
After the gridding model is established, the resistivity needs to be filled in each grid, and an initial resistivity model is established. Filling the same constant resistivity for each grid, and directly taking the uniform resistivity model as an initial resistivity model; and a second method is that inversion is carried out by using the data outside the inversion region in the step 2, the inversion result is used as an initial resistivity model in the next step, and prior information can be added in the step to carry out constraint inversion.
Step 5, frequency division chromatography inversion:
1) first round of tomographic inversion: inputting a highest frequency data set and an initial resistivity model, and performing inversion to obtain a first round of chromatographic inversion model;
2) second round of tomographic inversion: inputting an adjacent lower frequency data group and a previous chromatographic inversion model, and inverting to obtain a new chromatographic inversion model by taking the grid resistivity in the effective exploration depth of the previous group of data as constraint;
3) and (3) repeating the step 2) until all the grouped data are inverted, and obtaining a final inversion model, namely a resistivity model.
Step 6. inversion result evaluation
Evaluating the inversion result according to the prior data of the existing drilling, well logging, geological outcrop and the like, and finishing the inversion if the inversion result accords with the prior data and a satisfactory resolution is obtained; otherwise, starting from step 2, the frequency data packet is readjusted and steps 2 to 6 are repeated.
A magnetotelluric measuring line in a certain place in the western part of China is selected to carry out an experiment, the measuring line is 49 kilometers long and is 57 observation points, each observation point has 38 frequency point data, the frequency range is from 0.001Hz to 320Hz, and the inversion depth is 10 km. FIG. 1 is a conventional inversion result, firstly, a grid is generated in an inversion target area, and the thickness of the grid is gradually increased from shallow to deep; then filling each grid with uniform resistivity; then inputting all 38 frequency point data for inversion. It can be seen that the overall resolution is low, and in particular, deep structures below 5km are in the shape of lumps, which is inconsistent with the geological knowledge of the region. 38 frequency points are divided into 4 groups according to effective exploration depth, frequency division tomography inversion is carried out, and FIG. 2 shows inversion results of the method, so that a structural mode above 5km is basically consistent with a conventional inversion structure, but the resolution is obviously improved, a layered structure below 5km appears, a set of low-resistance stratum is clearly disclosed to be clamped in the middle of the high-resistance stratum and is consistent with geological knowledge of the region, and the method proves that the resolution and the reliability of deep inversion are effectively improved.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
Claims (8)
1. The magnetotelluric frequency division tomography inversion method is characterized by comprising the following steps of: estimating effective exploration depth according to the observation point frequency point data; grouping the frequency point data; establishing a gridding model; constructing an initial resistivity model; frequency division chromatography inversion; and (4) evaluating an inversion result.
2. The method of claim 1, wherein the effective depth of investigation is estimated for all frequency bin data for all observation points according to the following formula:
wherein rho is apparent resistivity, and f is frequency corresponding to the apparent resistivity.
3. The method of claim 1, wherein the frequency point data is grouped as follows: firstly, according to the effective exploration depth and the determined inversion depth, frequency point data are divided into two groups of data in an inversion region and data outside the inversion region; and secondly, grouping the data in the inversion region according to the effective exploration depth.
4. The method according to claim 1, wherein during the gridding model building process, the mesh of the inversion target area is divided, the two-dimensional inversion division mesh is rectangular, the three-dimensional inversion division mesh is a vertical hexahedron, the mesh can be uniform mesh or non-uniform mesh, and the relief topography can be depicted in an encrypted division manner at the earth surface.
5. The method of claim 4, wherein the number of the grouped data is m, and the number of the meshes divided from the effective exploration depth corresponding to the previous group of data with a higher frequency than the grouped data to the effective exploration depth corresponding to the grouped data is n, wherein m and n should satisfy m/n > 0.2.
6. The method of claim 1, wherein the initial resistivity model is constructed by: filling the same constant resistivity for each grid, and directly taking the uniform resistivity model as an initial resistivity model; alternatively, the first and second electrodes may be,
and (4) performing inversion by using the data outside the inversion region, taking the inversion result as an initial resistivity model of the next step, and adding prior information to perform constraint inversion in the step.
7. The method of claim 1, wherein the frequency division tomography inversion comprises the following specific steps:
1) first round of tomographic inversion: inputting a highest frequency data set and an initial resistivity model, and performing inversion to obtain a first round of chromatographic inversion model;
2) second round of tomographic inversion: inputting an adjacent lower frequency data group and a previous chromatographic inversion model, and taking the grid resistivity in the effective exploration depth of the previous group of data as constraint to invert to obtain a new chromatographic inversion model;
3) and (3) repeating the step 2) until all the grouped data are inverted, and obtaining a final inversion model, namely a resistivity model.
8. The method of claim 1, wherein the inversion result is evaluated according to the previous verification data, and if the inversion result meets the previous verification data and a satisfactory resolution is obtained, the inversion is ended; otherwise, the frequency data packet is readjusted to continue inversion.
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